The detection of intrusions using sound or vibration sensing devices is well-known per se. For instance, U.S. Pat. No. 3,618,062 to Ott shows in FIG. 1A a plate having a piezoceramic disk fixed thereto and responsive to vibration of the plate resulting from an intrusion as measured by the transducer when the plate is attached to the wall of a room. FIG. 3 of that patent shows an assembly in which the mounting plate is parallel to the wall and clamped thereto at two spaced points.
U.S. Pat. No. 2,735,025 to Wiggins shows a piezoceramic plate which is supported at its two opposite ends by an L-shaped bracket, the device being operative to generate an electric potential proportional to deflections of the ceramic plate when the bracket is flexed.
Other patents showing beams carrying a ceramic plate and sensitive to deflections thereof include the phonograph pickup device shown in U.S. Pat. No. 3,144,522 to Bernstein and the surface irregularity measuring device shown in U.S. Pat. No. 2,728,222 to Becker et al.